1. Field of the Invention
The present invention relates to a magneto-optical medium for reproduction of information by utilizing domain wall displacement, and a method of reproduction of information.
2. Related Background Art
In recent years, the magneto-optical mediums are attracting attention as a rewritable high-density recording medium: the magneto-optical medium which records information in magnetic domains of a magnetic thin film utilizing thermal energy of a semiconductor laser and reads out the recorded information by utilizing a magneto-optical effect. The treatment data are becoming diversified into sounds, pictures, animations, and so forth, and the data size thereof is increasing. Therefore, higher recording density and higher recording capacity are required for the magneto-optical mediums.
Generally, the line recording density of the magneto-optical medium depends largely on the laser wavelength of the reproducing optical system and the numerical aperture of the lens NA. Since the beam waist diameter depends on the laser wavelength xcex of the reproducing optical system and the numerical aperture NA of the lens, the spatial frequency of the signal-reproducing recording pits is limited to about 2NA/xcex. Therefore, for realizing the high density of a conventional optical disk, the laser wavelength of the reproducing optical system should be shortened, or the numerical aperture of the objective lens should be increased. However, the shortening of the laser wavelength is difficult because of the efficiency and heat generation of the element. The increase of the numerical aperture of the objective lens causes the problem that the lens and the disk are brought extremely close, causing collision or a like mechanical problem.
To solve such problems, super-resolution techniques are being developed in which the recording density is increased by improvement of the constitution of the recording medium or by improvement of the reproduction process without changing the laser wavelength or the numerical aperture of the objective lens. For example, Japanese Patent Application Laid-Open No. 7-334887 discloses a super-resolution system in which a lamination structure is formed from a memory layer for memorizing recorded information, a reproducing layer for masking a part of a reproducing light spot area, and an intermediate layer for controlling the exchange coupling between the above layers: the recorded information is transferred to the reproducing layer by utilizing only a part of the reproducing light spot by the temperature distribution caused by irradiation of the reproducing light spot in the recording medium to reproduce the fine magnetic domain.
In the above system, a portion of the reproducing light spot is masked and the temperature gradient is utilized. In other words, the resolution power is raised by restricting the aperture for reading the recorded pitches to a smaller region substantially. Therefore, the masked portion of the light is ineffective, decreasing the amplitude of the reproduction signals. Since the light projected to the masked portion does not contribute for producing the information signals, the smaller aperture for higher resolution will decrease the effective light to lower the signal level disadvantageously.
For utilizing effectively the reproducing light without causing the above problems, Japanese Patent Application Laid-Open No. 6-290496 discloses a reproducing method in which a domain wall displacement layer having a lower domain wall coercivity is provided on the reproduction light-introducing side and the domain in the domain wall displacement layer is displaced toward a high temperature side by utilizing the temperature gradient in the reproducing light spot to enlarge and reproduce the domain within the spot. According to this reproduction method, even if the recorded mark (magnetic domain) is small, the signal is reproduced with enlargement of the domain to utilize effectively the reproducing light to raise the resolution power without decreasing the reproducing signal amplitude.
The reproduction process disclosed in the above Japanese Patent Application Laid-Open No. 6-290496 is explained in detail by reference to drawings.
FIGS. 6A to 6C are drawings for explanation of construction of the magneto-optical recording medium and the principle of information reproduction disclosed in the above Patent Laid-Open publication. FIG. 6A is a schematic cross-sectional view illustrating the constitution of the magneto-optical recording medium and the magnetization state of the portion irradiated by a reproducing light. FIG. 6B is a graph showing a temperature distribution in the magneto-optical recording medium on irradiation of the reproducing light beam. FIG. 6C is a graph showing the distribution of the domain wall energy density "sgr" of the domain wall displacement layer corresponding to the temperature distribution shown in FIG. 6B.
As shown in FIG. 6A, this magneto-optical recording medium has a recording layer constituted of magnetic layer 111 as a domain wall displacement layer, magnetic layer 112 as a switching layer, and a magnetic layer 113 as a recording layer, laminated successively. In this recording medium, magnetic layer 111 is formed on the side of introduction of the reproducing light beam. Arrows 114 indicate orientation of atomic spins in the layers. Magnetic walls 115 are formed at the interface of the regions where the orientation of the atomic spins are reversed. The signal waveform in the lower part of FIG. 6A shows the recorded signals corresponding to the magnetization state of this recording layer.
Arrow 118 indicates the direction of movement of the recording medium. With the movement of the recording layer in the direction of arrow 118, light beam spot 116 is moved relatively along the information track of the recording layer. At the portion irradiated with this light beam spot 116, the temperature distribution is caused such that the temperature rises from before the front of the light spot and the temperature peak is formed behind the light spot. In this example, the medium temperature T reaches the temperature Ts near the Curie temperature of magnetic layer 112 at the position Xs.
In magnetic layer 111, the domain wall energy density "sgr" distributes as shown in FIG. 6C in such a manner that the energy density is minimum near the point of temperature peak behind the light beam spot 116 and is higher before the light beam spot. The gradient of the domain wall energy density "sgr" along the position X direction causes exertion of the force F, shown by the equation below, to the domain walls at the position X in each of the layers.
F=∂"sgr"/∂X xe2x80x83xe2x80x83(1) 
This force F acts on the domain walls to move toward the lower domain wall energy side. Since magnetic layer 111 has a lower domain-wall coercivity and a higher domain-wall mobility, domain walls 115 are driven readily by this force F in this layer. However, in the region before position Xs in the front of the light spot, the medium temperature is lower than Ts, which prevents movement of domain wall 115 and fixes it at the position corresponding to the domain wall in magnetic layer 113 having a higher coercivity.
With tyis magneto-optical medium, with movement of domain walls 115 in the movement direction 118, the temperature of the portion of domain wall 115 of the medium rises to reach the temperature Ts near the Curie temperature of magnetic layer 112 at the position Xs. Thereby the exchange coupling between magnetic layer 111 and magnetic layer 113 is canceled. Consequently, domain wall 115 of magnetic layer 111 is driven instantaneously to the region of a higher temperature and a lower domain wall energy density. In FIG. 6A, this movement direction is shown by dotted arrow 117. After passage of domain walls 115 through under light beam spot 116, the atomic spins of magnetic layer 111 within the light spot are oriented in one and the same direction.
Every moment when domain wall 115 reaches position Xs with the movement of the medium, domain wall 115 is displaced instantaneously to cause orientation of the atomic spin in one and the same direction under the spot. As the results, the amplitude of the reproduction signals is maximized invariably. Thereby, the problems of waveform interference or the like caused by an optical diffraction limit are completely solved.
FIGS. 7A to 7H show the relation between the spot position and the reproduced signal in the information reproduction by the aforementioned domain wall displacement. For comparison, FIGS. 8A to 8H show the relation between the light spot position and the reproduced signal in the information reproduction without the aforementioned domain wall displacement. FIGS. 7A to 7G show the states of movement of reproducing light spot 131 on information tracks 136 having magnetic domains 133 formed which have various record mark lengths. FIG. 7H and FIG. 8H show the waveforms of the derived reproduced signals.
In the case where the domain wall displacement is not caused, the maximum amplitude of the reproduced signals can be obtained only when the reproducing light spot 131 fits entirely to one domain of information track 136 (see FIG. 8H). With the domains 133 smaller than the spot diameter, a part of reproducing spot 131, not entire thereof is introduced to the magnetic domain (see FIGS. 8C to 8G) to result in unclear reproducing signals (see FIG. 8H).
On the other hand, in the case where the domain wall is displaceable, reproducing spot 131 is moved in the reproducing spot movement direction 132, which allows the temperature profile to move in the same direction as shown in FIGS. 7A to 7G. The temperature is distributed in reproducing spot 131 such that the medium reaches the critical temperature Ts of magnetic layer 112 as shown in FIGS. 6A to 6C at the portion immediately before the spot in the spot movement direction. According to the above-described principle of the domain wall displacement, the temperature of the part of the domain wall 134 is raised to the critical temperature Ts directly before the reproducing spot 131 reaches domain wall 134, and domain wall 134 is displaced in the direction 135 of domain wall displacement, which direction is reverse to the direction 132 of the reproducing spot movement. As the result, the reproducing spot 131 enters the record mark (see FIG. 7B) to give instantaneously the maximum amplitude of the reproduction signal (see FIG. 7H). Furthermore, the domain walls are displaced successively in the domain wall displacement direction 135 with close approach of the reproducing spot 131 to the domain wall of the record mark (FIGS. 7C to 7G), which produces sharp reproduction signals (see FIG. 7H).
The above reproduction process of Japanese Patent application Laid-Open No. 6-290496 involves the problems below.
For obtaining the temperature gradient for domain wall displacement only by heating with the reproducing light beam, the peak of the temperature distribution will be formed within the reproducing light spot area. For example, as shown in FIG. 6B, the temperature rises from before the front of the light spot and the temperature peak is formed in the rear portion of the light spot relative to the beam movement direction. In this state, the domain wall displacement occurs not only before the front of the reproducing light spot but at the rear portion of the reproducing light spot. Thus the both domain wall displacements are read out by the reproducing light spot, not giving satisfactory reproduction signals, disadvantageously. This Patent Laid-Open Publication describes a provision of a means for obtaining a desired temperature distribution separately from the reproducing light beam. Although this means can prevent the domain wall displacement from the rear portion of the reproduction light spot to some extent, the means for the desired temperature distribution complicates the reproduction system.
For solving the above problem of the domain wall displacement from behind the reproduction light spot, Japanese Patent Application Laid-Open No. 11-86372 discloses a method for suppressing the displacement behind the spot by application of a regeneration magnetic field. However, the required reproducing magnetic field-generating means also complicates the reproduction system.
The present invention intends to provide a magneto-optical recording medium and an information reproduction process which enables high density signal reproduction surpassing the resolving power of an optical system, and provides also an information reproduction method employing the magneto-optical recording medium.
The magneto-optical medium of domain wall displacement type of the present invention comprises:
a domain wall displacement layer in which a domain wall is displaceable;
a memory layer which stores recorded domains in accordance with information; and
a switching layer which is provided between the domain wall displacement layer and the memory layer, having a Curie temperature lower than Curie temperatures of the domain wall displacement layer and the memory layer, and having a boundary temperature higher than room temperature for transformation from a perpendicular magnetization film to an in-plane magnetization film.
The reproduction process of the present invention comprises the steps of:
causing a prescribed temperature distribution having a temperature range higher than Curie temperature of the switching layer on the magneto-optical medium;
canceling exchange coupling between the domain wall displacement layer and the memory layer in a temperature range higher than the Curie temperature of the switching layer, and displacing a domain wall formed in the domain wall displacement layer to a high-temperature side by a temperature gradient of the temperature distribution;
keeping the switching layer in a state of an in-plane magnetization film, in a course of cooling of the switching layer below the Curie temperature, in a region where the orientation of magnetization of a transition metal sublattice of the domain wall displacement layer and the orientation of magnetization of a transition metal sublattice of the memory layer are reversed, while the medium temperature in the region falls from the Curie temperature to the prescribed temperature higher than room temperature; and
transforming the switching layer in the region to a state of a perpendicular magnetization film at a temperature below the prescribed temperature.